Preheat oven to 375°F and line 2 large baking sheets with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and brown sugar together on medium speed until light and fluffy, about 2 minutes.
½ cup unsalted butter, 1 cup brown sugar
Add the egg and vanilla and mix until fully incorporated. Scrape down the sides of the bowl as needed.
1 large egg, 1 teaspoon pure vanilla extract
Add the flour and baking soda and mix just to combine — make sure not to overmix!
¾ cup all-purpose flour, ¾ teaspoon baking soda
Add the oats and mix until combined.
1½ cups old-fashioned oats
Lastly, add the raisins and fold them in gently.
½ cup raisins
Scoop 2-tablespoon-sized balls of cookie dough onto the prepared baking sheets, making sure to scoop them at least 2 inches apart.
Bake the cookies for 12-14 minutes, or until they’re golden brown around the edges.
Allow the cookies to cool to room temperature on the baking sheet (they’ll harden slightly as they cool).